The conventional bridgewire employed in a squib or igniter is required to be in intimate contact with the pyrotechnic material for reliable ignition. To ensure the presence of pyrotechnic in the proximity of the bridgewire, at least in the manufacturing process, the pyrotechnic is spotted or beaded on the bridgewire with any additional amount of pyrotechnic being uniformly loaded in proximity to the beaded bridgewire. With adverse environment applied to the squib, the bridgewire-pyro-interface may change. Such changes may include cracks, air bubbles or voids adjacent to the bridgewire thus causing the bridge to "burn" in the area of no pyrotechnic. This will cause increase delay times and possibly failures to the point of no ignition. Even without severe adverse environmental exposure functional delay times at some given electrical current level can be a problem, even for uniformly loaded squibs. For example, squibs which are consistently uniformly loaded may exhibit a delay time of several milliseconds with 5.0 amps of current.
Special requirements have placed additional demands on the squib or initiator designer. For example, one such requirement has been a 1 watt/1 amp no-fire characteristic initiator of the type required for initiating action of a destruct unit for rocket motor cases for weather rockets. This initiator could be mass produced to a thinness of about 0.070 inch which made it ideally suited for fitting into the approximate 0.100 inch space envelope of the honeycomb of a rocket motor case. A one watt/one amp no-fire match type initiator is described and claimed in U.S. Pat. No. 3,910,188 issued to Philip M. Stevens on Oct. 7, 1975 and assigned to the United States of America as represented by the Secretary of the Army, Washington, D.C. This match type initiator is comprised of a Pyrofuze bridgewire soldered between a bared length of the conductors of a dual conductor flat cable and covered with an ignition material with about 90% by weight of the material being comprised of lead thiocyanate, potassium chlorate, and charcoal and with about 10% by weight of the material being a binder. An external epoxy adhesive coating seals the initiator thereby obviating the requirement of an initiator case.
There exists a need for a highly reliable squib which can employ proven components and proven means for assembling. Additionally, there is a need for a highly reliable squib which employs proven components and proven means for assembling while reducing the controls on loading and assembling procedures.
Therefore, an object of this invention is to provide a squib or initiator that has improved reliability, while reducing the controls on loading the pyrotechnic and assembling procedures.
Another object of this invention is to provide a squib that employs a bimetallic bridgewire in combination with loose pyrotechnics, propellants and mixtures of exothermic materials.
A further object of this invention is to provide a squib that employs a bimetallic bridgewire that exhibits the characteristic of throwing out constituents from an alloying reaction which proceeds violently and exothermically following the application of electric current. The throwing out of constituents being within a confined volume of relatively small proportions and occuring in the vicinity of a loading of an easily-ignitable pyrotechnic or a powdered double-base propellant, single-base propellant, or a powdered composite propellant permits less control of the loading and assembling procedures while retaining improved reliability.